The United States telecommunications industry is in a state of transition. During the next several years, it is expected that the monopoly held by local exchange carriers will be substantially altered, and competitive access providers will begin to offer customer service for both toll, and local exchange calls. To facilitate this transition without creating an excessive burden on subscribers of a competitive access provider, a requirement for service provider number portability is anticipated. Service provider number portability is defined as a change in service provider without a change in directory number. Eventually, location number portability will also be available so that customers can move to other geographic locations without a change in directory number.
Number portability, regardless of type, significantly impacts call routing in the existing telecommunications network. This is because calls between telephone customers in the United States are established on the basis of a national directory number plan, wherein each customer line is identified by a 10-digit directory number having geographic, and central office switch significance. The 10-digit directory number comprises a three-digit area code (generally denoted by the letters "NPA") defining a specific geographic region, followed by a three-digit office code (denoted by the letters "NXX") identifying a particular central office switch in the network. The remaining four digits are customer line identifiers (denoted by the letters "XXXX") for establishing the line address of a particular customer line served by the central office switch identified by the NXX digits. Customers who change service providers while retaining their directory number are typically served by a central office switch which normally serves NPA-NXX digits different than the NPA-NXX digits of the customer's directory number. In other words, such customers have "ported" their directory numbers.
Ported directory numbers require special accommodations in the existing telecommunications network. One example of such an accommodation is the implementation of a location routing number (LRN) scheme in which each central office switch in the public-switched telephone network is identified by a unique set of "NPA-NXX" digits. Number portability databases store LRN data, and are accessed to ensure proper routing of calls to ported directory numbers. A detailed explanation of LRNs, and call routing may be found in U.S. Pat. No. 5,475,749, assigned to AT&T.
The routing of calls to ported directory numbers is also affected when "call blocking" (also known as "code blocking") is initiated. Call blocking is a procedure in which only a portion of all telephone calls to a particular destination (e.g., a central office switch or directory number) is completed in accordance with a predetermined blocking scheme. Call blocking is initiated by network management personnel to control high call volumes to a particular destination. For example, call blocking may occur as a result of a natural disaster affecting a particular geographic region. Central office switches located in the affected region may be controlled by a block which allows only one call per second to terminate to customer lines served by the switch, wherein the normal termination rate is two calls per second. This type of blocking is used to reduce the high incoming call volume to affected switches so that subscribers served by the switch can use network resources to complete outgoing calls. The blocked calls receive either an "all circuits busy" announcement, or a fast busy signal.
Individuals who retain directory numbers ported from another switch are adversely affected by existing call blocking procedures directed to central office switches. This is because blocking an entire central office switch is based on "NPA-NXX" digits which identify the switch. In other words, incoming call volume is decreased by completing only a portion of the number of calls directed to directory numbers including blocked "NPA-NXX" digits. Unfortunately, a ported directory number customer who happens to retain "NPA-NXX" digits of a central office switch under call blocking controls will also have calls blocked. Therefore, there is a need in the art to exclude ported directory number customers adversely affected by call blocking controls.